Little Knowledge: centrifugal pump cavitation phenomenon and the installation he
1, Centrifugal pump The cavitation phenomenon
Centrifugal pump cavitation phenomenon is being transported in the transportation of liquid as the temperature is equal to or lower than the saturation vapor pressure pump entrance (actually the entrance for the leaves) and some of the pressure of vaporization, causing the pump noise and vibration, serious, pump flow, pressure head and efficiency decreased significantly, obviously, is pump cavitation phenomenon occurs not allow normal operation. To avoid cavitation phenomenon is the key to the correct height of the pump, especially when the transmission fluid temperature is high volatile, the more to note.
2, centrifugal pump mounting height Hg
Allow smoking on the high vacuum pump Hs refers to the pressure p1 at the entrance to reach the maximum allowable vacuum
And practical to allow a high degree of vacuum suction on the Hs values are not calculated according to the value, but by the pump manufacturer experimental determination of the value that is attached to the pump samples for user reference. Bit should be noted that pump the sample values are given in Hs with water as the working medium, and the operating conditions of 20 and pressure of 1.013 × 105Pa when the value, when the operating conditions and the working medium is not the same time, the need for conversion.
(1) transport of water, but the operating conditions and experimental conditions, Ke Yi-type conversion under
Hs1 = Hs + (Ha-10.33) – (H -0.24)
(2) transport of other liquid when the liquid is transferred and the villain and the experimental conditions, conditions are not the same time, the need for two-step conversion: the first step according to the type will be detected sample pump Hs1; second step according to the following formula will be converted into Hs Hs1
2 cavitation margin h For Pump , When the installation height calculation used to calculate the cavitation margin h, that
With NPSH h check samples taken from the pump, and its value is also measured with 20 water for. If the delivery of other fluids, should also be calibrated, detailed investigation of the books.
From Security Point of view, the pump mounting height should be less than the calculated values. Also, when calculating the Hg is negative, indicating suction nozzle location should be in the tank below the surface.
Case of a centrifugal pump from the sample 2-3 were allowed to check on the vacuum suction height of Hs = 5.7m. Known inhalation Pipeline All resistance was 1.5mH2O, the local atmospheric pressure is 9.81 × 104Pa, the liquid in the suction line in the dynamic pressure can be neglected. Calculate:
(1) 20 water for delivery when the pump installation;
(2) to transport 80 water pump mounting height.
Solution: (1) 20 water for delivery when the pump mounting height
Known: Hs = 5.7m Hf0-1 = 1.5m U12/2g 0 Local atmospheric pressure is 9.81 × 104Pa, and the pump factory basically consistent with experimental conditions, so the installation height of the pump
Hg = 5.7-0-1.5 = 4.2m.
(2) transport 80 water pump mounting height
80 water delivery, not directly in the sample using the pump mounting height Hs value, Hs need to press on when the line type conversion, that is,
Hs1 = Hs + (Ha-10.33) – (H -0.24)
Known Ha = 9.81 × 104Pa 10mH2O, from the appendix to look up the saturated vapor pressure of water 80 to 47.4kPa.
Hv = 47.4 × 103Pa = 4.83mH2O
Hs1 = 5.7 +10-10.33-4.83 +0.24 = 0.78m
Hs1 value will be substituted into the type of installation height obtained
Hg = Hs1-Hf0-1 = 0.78-1.5 =- 0.72m
Hg is negative, that the pump should be installed below the liquid level in the pool, or at least lower than the surface 0.72m.
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